The Surfactant Effect On the Dynamics of Drop Coalescence

Drop coalescence is pervasive in nature, as in the coalescence process

that produces raindrops, and is a key component in important

technological processes such as spray, combustion, food processing and

propulsion systems. Although surfactants are ubiquitous as

contaminants and frequently used as industrial additives, little is

known about the coalescence of surfactant-laden drops, largely due to

the complex interplay of viscous, inertia, capillary and Marangoni

forces involved. Here we report direct numerical simulations that

enable a better understanding of the physical mechanisms of

coalescence of surfactant-laden drops at low Reynolds numbers.

Simulations reveal that the presence of surfactant modifies the rate

of coalescence through the interplay between capillary forces weakened

by surfactant accumulating on the joining meniscus, and the growing

strength of  Marangoni forces induced by surfactant concentration

gradients. Results from this study can have a significant impact on

the more rational design of surfactant additives for spray and

propulsion systems.